I. A conventional water purifying apparatus for producing ultrapure water from raw water such as city water, well water and industrial water is composed of a pretreatment equipment, a primary pure water equipment and secondary pure water equipment. The pretreatment equipment is composed of coagulation, floatation and filtration units. The primary pure water equipment is composed of two reverse osmosis membrane units and a mixed bed ion exchange unit, or a demineralization equipment and a reverse osmosis membrane unit. The secondary pure water equipment is composed of a low pressure mercury ultraviolet oxidization unit, a demineralization equipment and an ultrafilter (ultrafiltration membrane separation device).
Either the following process (i) or (ii) is adopted.
(i) Raw water.fwdarw.coagulation, floatation, filtration units.fwdarw.first reverse osmosis membrane unit.fwdarw.second reverse osmosis membranes unit.fwdarw.mixed bed ion exchange unit.fwdarw.primary ;pure water storage tank.fwdarw.low pressure mercury ultraviolet oxidization unit.fwdarw.demineralization equipment.fwdarw.ultrafilter.fwdarw. PA1 (ii) Raw water.fwdarw.coagulation, floatation, filtration units.fwdarw.ion exchange equipment [two-bed three-tower type (cation exchange tower.fwdarw.vacuum degas tower or decarbonation tower.fwdarw.anion exchange tower) or four-bed five tower type (first cation exchange tower.fwdarw.second anion exchange tower.fwdarw.vacuum degas tower or decarbonation tower.fwdarw.second cation exchange tower.fwdarw.second anion exchange tower )].fwdarw.reverse osmosis membrane unit.fwdarw.primary pure water storage tank.fwdarw.low pressure mercury ultraviolet oxidization unit.fwdarw.mixed bed ion exchange unit.fwdarw.ultrafilter.fwdarw.ultrapure water PA1 pH: 2 to 4 PA1 Acid component: hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid PA1 Alkali component: ammonia PA1 TOC: 1 to 3 ppm PA1 [TOC component: isopropyl alcohol, methanol, acetone, surfactant] PA1 H.sub.2 O.sub.2 : about 20 ppm PA1 (a) Since the amount of TOC in raw water is large (2 to 3 ppm), when H.sub.2 O.sub.2 in the raw water is removed, slime is produced on the first activated carbon. PA1 (b) The slime produced on the first activated carbon is brought into the reverse osmosis membrane unit, and contaminates the reverse osmosis membrane, thereby making the stable operation of the reverse osmosis membrane equipment difficult. PA1 (c) When a polyamide membrane (PA membrane) which has a high removal rate with respect to alcohol, especially, isopropyl alcohol is used as the membrane of a reverse osmosis membrane unit (RO unit), since the PA membrane has no resistance to free chlorine, it is impossible to add an oxidizing agent such as NaClO to feed water so as to prevent the slime contamination. PA1 (d) If a cellulose acetate membrane (CA membrane) which has resistance to free chlorine is used as the membrane of a reverse osmosis membrane unit, since the alcohol removal rate is low, the load of the UV oxidation unit provided at a subsequent stage is increased, which leads to a rise in the cost. PA1 (e) The water recovery of the apparatus as a whole is low. Since the water recovery of the reverse osmosis unit is 90% at its maximum, the water recovery of the apparatus as a whole is not more than 90%. In other words, the loss is not less than 10%. PA1 (f) If the quality of the raw water at the time of designing the apparatus is deteriorated (increase in TOC), the apparatus cannot be used. PA1 (g) Since the recovery system is complicated, the maintenance requires much labor. PA1 Urea-containing water: The city water of Atsugi City, Kanagawa prefecture, Japan with urea added thereto so that the urea concentration was 500 ppb. PA1 Heating temperature: 135.degree. C., 155.degree. C. PA1 No. 1 pH 5.5, 20 ppm of K.sub.2 S.sub.2 O.sub.8 PA1 No. 2 pH 4.5, 20 ppm of K.sub.2 S.sub.2 O.sub.8 PA1 No. 3 pH 5.5, 10 ppm of H.sub.2 O.sub.2 PA1 No. 4 pH 4.5, 10 ppm of H.sub.2 O.sub.2
ultrapure water
The above deminerlization equipment comprises an anion exchange column, a mixed bed ion exchange column or an anion exchange column and a mixed bed ion exchange column which are arranged in series.
Among these units and equipments, the reverse osmosis membrane units, the demineralization equipment and the low pressure mercury ultraviolet oxidization unit reduce the TOC components in raw water by separation, adsorption, decomposition or the like.
TOC reducing-mechanisms of these units or equipmemts are as follows.
Reverse osmosis membrane unit: Raw water is filtered through a reverse osmosis membrane so as to remove ionic and colloidal TOC.
Low pressure mercury oxidization unit: TOC is decomposed into an organic acid and further into CO.sub.2 by ultraviolet of 185 nm projected from a low pressure mercury ultraviolet lamp. The organic acid produced is removed by an ion exchange resin provided at a subsequent stage. This mechanism is used especially for the decomposition of a volatile organic matter.
The ultrapure water produced by the above-described conventional pure water apparatus, however, still contains about 3 to 5 ppb of TOC.
In order to further reduce the TOC in ultrapure water, the ultrapure water may be treated by a multiplicity of stages of reverse osmosis membrane units and an increase in irradiation of a low pressure mercury ultraviolet unit be considered. It has been confirmed, however, as a result of the studies undertaken by the inventors that neither can further reduce the TOC in ultrapure water because of the following reasons.
Raw water contains a urea component as one of the TOC components. Since the urea component is nonionic, the urea component is impossible to be removed by ion exchange. The urea component is removed at a low rate by a reverse osmosis membrane separation. For example, the urea component removal rate by a high salt rejection type reverse osmosis membrane unit, which is generally used as one of the two reverse osmosis membrane units in a primary pure water purifying equipment is about 60%. In addition, urea cannot be decomposed by a low pressure mercury ultraviolet oxidization unit.
Raw water contains not a little amount of urea, which obstructs the reduction of the TOC in ultrapure water. Since it is difficult to remove urea by a conventional water purifying apparatus, there is a limitation to the reduction of TOC.
For example, the city water of Atsugi City, Kanagawa, Japan contains about 30 ppb of urea. Even after treatment of the city water by the above-mentioned process (ii) which uses two reverse osmosis membrane units as the primary pure water equipment, about 8 ppb of urea is detected in the ultrapure water obtained by a conventional ultra water apparatus. 8 ppb of urea will come to about 1.5 ppb as calculated in terms of TOC. It is about 50% of the TOC components in ultrapure water which contains 3 ppb of TOC.
II. Waste water from a semiconductor rinsing system is treated so as to produce ultrapure water and be used again as semiconductor rinse water.
The properties of the waste water of the semiconductor rinsing system are as follows:
In order to treat such waste semiconductor rinse water, the process comprising the following steps is adopted.
______________________________________ First activated carbon adsorption (removal of H.sub.2 O.sub.2 and surfactant) .dwnarw. Weak anion exchange resin .dwnarw. Strong cation exchange resin .dwnarw. Strong anion exchange reisn .dwnarw. Reverse osmosis (separation of TOC components) .dwnarw. Addition of oxidizing agent (H.sub.2 O.sub.2) .dwnarw. Projection of high pressure mercury ultraviolet (UV) (Decomposition of TOC components) .dwnarw. Second activated carbon adsorption (Removal of unreacted H.sub.2 O.sub.2) .dwnarw. Strong anion exchange reisn .dwnarw. Treated water ______________________________________
About 90% of TOC components is removed by this process.
This process, however, has the following defects.
If the quality of the raw water is deteriorated, there is no alternative but to add a high pressure UV lap and a UV oxidation tank. However, since it takes a long time to add a high pressure UV lamp, it is impossible to treat the water instantly.